The advance of technology in the end of 20th century, especially in micro-technologies, including micro-mechanical systems, communication and control devices, made it feasible to design and manufacture unmanned aircraft capable of performing various military and civilian missions. In 2001, about 50 US companies, academic institutions, and government organizations have been developing over 150 UAV designs. About 115 of these designs are flying, while some 26 models of UAV's are in exploitation or ready for production. Most of the UAV used today are in fact small airplanes measuring 2 m and more wingspan and capable to operate at tens and hundreds of miles range.
In the 1990s, new classes of UAV started to develop: Mini and Micro-UAV. Mini-UAV are vehicles of about 20 cm to 1.2 m size while Micro-UAV are limited to 6 inches (15 cm) in either dimension, according to the definition of Defense Advanced Research Project Agency (DARPA) of the USA. The aerodynamic design of such small vehicles is not tractable by well-developed methods applied to conventional aircraft, mainly due to the low Reynolds number of their flight (2*104-3*105), and the requirement of low flight speed, which is comparable with moderate wind speeds, such as 10-20 m/s.
An example of Mini-UAV is the “Sender” of the US Naval Research Laboratory. It has a classical rectangular wing, tractor propeller and V-like empennage. The wingspan is 1.2 m and the flight weight is 4.5 kg. The vehicle is launched from a catapult and has near 100-mile range capability.
Very intensive research and design effort takes place in the Micro-UAV (MAV) applications. From the late 1990s, American universities stage yearly competitions of MAV, while governmental and commercial laboratories pursue projects of mission-worthy vehicles.
MicroSTAR MAV of Sanders AS&T group and Lockheed is an operable prototype with flying wing configuration. The air vehicle has delta wing with 9-inch span, side panels and tractor propeller driven by an electric motor. It is capable to carry miniature video and IR cameras, communication and control means in a 2-mile range.
Intelligent Automation, Inc has designed and fabricated a 15 cm, 90 g MAV designed to fly at 40 mph for about 20 min, carrying a video camera, two-way RF communication receiver, and servomotors. The MAV has classical wing arrangement with swept wings, horizontal stabilizers, vertical keel, and tractor propeller driven by an internal-combustion engine.
The above-mentioned competitions and academia projects have brought up other aerodynamic designs seeking greater lift at reduced flight speed acceptable stability and controllability:                Mississippi State University “SKYDOG” with triplane wing configuration, two keels, horizontal stabilizer connecting the keels, and tractor propeller;        University of Notre Dame MAV with flying wing of inverse Zimmerman planform (squared-off), central vertical keel and tractor propeller;        University of Florida MAV with biplane tailless configuration. Two vertical fins connect the two wings at ¾ of the semi-span from the center plane. The upper wing is displaced about half-chord to the rear.        
A number of projects make use of the known fact in the aerodynamic design that double-wing configurations, such as biplane and tandem, provide greater lift and reduced induced drag in comparison to a single wing configuration.
The “Outrider” UAV of Alliant Techsystems has two essentially straight tandem wings with 3.30 m span, with the forewing above and the aft wing below the fuselage. Side panels to form a rigid frame connect the wingtips. The vehicle has a T-like empennage.
DE 43 32 867 suggests a similar configuration but closer to biplane because the two wings are also overlapping each other.
U.S. Des. Pat. No. 166,843 suggest a light aircraft tandem configuration with two swept wings, the forewing being above the fuselage, and a pushing propeller. The aircraft has two vertical keels with rudders behind the aft wing.
Small vehicle size must be convenient to operate in close proximity to a point of interest without being detected, loitering for 0.5-1 hour, and then returning. The aircraft must be able to fly in turbulent and high winds speed, to provide reasonable rate of climb, tight turns and be stable enough to serve as an airborne video platform. The aircraft must be easy enough to operate, so that soldiers may deploy it in the field with minimal training.
There is a clear conflict between constrains imposed on dimensions of Micro and Mini UAV and requirements to fly at reduced airspeeds with acceptable flying qualities and maneuver capabilities. Typical configurations that try to achieve maximum exploitation of the allowable area (rectangular, delta, ogive wings, etc.), suffer from poor aerodynamic efficiency, low values of maximum lift and problematic stability and control characteristics. The flight of these types of configurations relies massively on sophisticated flight control, while their performance is penalized by excessive weight of the battery (electric propulsion case) that should be carried for overcoming its poor aerodynamic efficiency.
Typical configurations of MUAV use overpowered engines for overcoming its poor aerodynamics efficiency and suffer from excessive weights of batteries, engine plant and airframe. In flight, this results in increased air speed, which harms loitering performance, take off/landing characteristics and quality of video display.